Toxicity of substrate-bound amyloid peptides on vascular smooth muscle cells is enhanced by homocysteine

Τhe main component of cerebral amyloid angiopathy (CAA) in Alzheimer's disease is the amyloid-β protein (Aβ), a 4-kDa polypeptide derived from the β-amyloid protein precursor (APP). The accumulation of Aβ in the basement membrane has been implicated in the degeneration of adjacent vascular smooth muscle cells (VSMC). However, the mechanism of Aβ toxicity is still unclear. In this study, we examined the effect of substrate-bound Aβ on VSMC in culture. The use of substrate-bound proteins in cell culture mimics presentation of the proteins to cells as if bound to the basement membrane. Substrate-bound Aβ peptides were found to be toxic to the cells and to increase the rate of cell death. This toxicity was dependent on the length of time the peptide was allowed to ‘age’, a process by which Aβ is induced to aggregate over several hours to days. Oxidative stress via hydrogen peroxide (H2O2) release was not involved in the toxic effect, as no decrease in toxicity was observed in the presence of catalase. However, substrate-bound Aβ significantly reduced cell adhesion compared to cells grown on plastic alone, indicating that cell–substrate adhesion may be important in maintaining cell viability. Aβ also caused an increase in the number of apoptotic cells. This increase in apoptosis was accompanied by activation of caspase-3. Homocysteine, a known risk factor for cerebrovascular disease, increased Aβ-induced toxicity and caspase-3 activation in a dose-dependent manner. These studies suggest that Aβ may activate apoptotic pathways to cause loss of VSMC in CAA by inhibiting cell–substrate interactions. Our studies also suggest that homocysteine, a known risk factor for other cardiovascular diseases, could also be a risk factor for hemorrhagic stroke associated with CAA.